I am a Linux Mint user. I have set up Thunderbird to check my Gmail account and download new messages via POP.
Then I made a mistake. I changed my setting is Google and “Turned Off” access for less secure apps. Well guess what, Google considers Thunderbird on Linux a “less secure app.”
When I fired up Thunderbird, it couldn’t login to my Gmail account. When I went and checked Gmail in Chrome I had gotten an mail saying “Sign-in attempt prevented.” The email explained that “Someone”, which was me, “just tried to sign in to your Google Account… from an app that doesn’t meet modern security standards.” I am sure it is just the Linux version of Thunderbird that “doesn’t meet modern security standards.”
Anyway, the way to fix it is to go back into your security settings and “Turn On” access for less secure apps. Below is the link to do that.